1. Technical Field
The present invention relates generally to liquid crystal (LC) phase-retarders realized by a liquid crystal film structure having one or more phase retardation regions formed therein, with each region having an optical axis and a phase retardation specified by the direction and depth of orientation of liquid crystal molecules along the surface of the liquid crystal film structure, and more particularly relates to linear cholesteric liquid crystal (CLC) polarizers realized by forming one or more phase retardation regions within CLC film structures.
2. Brief Description of the State of the Art
In the contemporary period, there is a great need to modulate the spatial intensity of light in liquid crystal display (LCD) panels, optical computing systems, holographic information storage and retrieval systems and the like. In nearly all such optical applications, linear polarizers are required to carry out this light intensity modulation function. In one form or another, a pair of linear polarizers are rotated relative to each other to produce a filter structure having a particular light transmittivity.
For example, in LCD panels, light produced by a backlighting structure (e.g. fluorescent tubes) is intensity modulated over pixel-sized regions by polarization-controlled pixel elements realized over the surface of the LCD panel. In particular, each pixel element is typically realized by interposing liquid crystal material between a pair of optically transparent electrodes which are connected to a computer-controlled voltage source response to digital image data sets corresponding to color or gray-scale images to be visually displayed. This liquid crystal structure is then disposed between a pair of linear polarizing filters. When a voltage level is impressed across the electrodes of each pixel element, an electric field is produced thereacross, causing the polarization direction of light transmitted from the first linear polarizing filter to rotate an amount proportional to the electric field strength and the light exiting from the second linear polarizing filter to be reduced in intensity. Accordingly, by simply controlling the electric field strength across the linear polarizing filter elements at each pixel element in an LCD panel, it is possible to control the intensity of light transmitted therefrom and thus form images at the surface of the LCD panel.
Presently, there exist two different types of polarizers, namely: dichroic (i.e. sheet) linear polarizers which operative upon an absorptive mechanism; and cholesteric liquid crystal (CLC) naturally-circular polarizers which operate upon a non-absorptive mechanism. It will be useful to briefly describe each of these linear polarizing structures below.
Dichroic linear polarizers were first invented by Edwin Land back in the early 1940's. This type of linear polarizing structure is based on a mechanism which absorbs and converts into heat the component of incident light along a first polarization direction P1, while transmitting without energy absorption the component of incident light along the desired orthogonal polarization direction, P2. Typically, such inefficient conversion of photonic energy results in the production of heat over the surface of the polarizer, causing undesired changes in the polarization characteristics of the polarizer, and at high intensities of incident light, the destruction of the polarizer. Also, by virtue of the inherent inefficiency of this type of polarizer, the use of dichroic polarizers in the construction of prior art LCD panels causes an inherent reduction in brightness by a factor of at least 50 percent. Yet, notwithstanding such shortcomings and drawbacks, the fact that dichroic linear polarizer can be produced in large surface areas and at low cost and weight, has lead to widespread use in LCDs and thus the proliferation of laptop computers.
In recent times, super broadband and narrow-band CLC-based linear polarizers have been developed for use in various optical applications. Exemplary structures can be found in the following publications: International Application Serial No. PCT/US96/17464 entitled “Super Broad-band Polarizing Reflective Material”, by Sadeg M. Faris, et al., published under International Publication Number WO 97/16762 on May 9, 1997; and EPO Application No. 94200026.6 entitled “Cholesteric Polarizer and Manufacture Thereof”. Both of these publications are incorporated herein by reference as if set forth in their entirety.
One of the principal advantages of both narrow and broadband cholesteric polarizers alike is that such polarizers make it possible to very efficiently convert unpolarized light into circularly polarized light without the undesired absorption of photonic energy, characteristic of dichroic polarizers. The reason for this advantage is that narrow-band and broadband CLC films alike exhibit polarization and wavelength dependent reflection properties by virtue of the helical ordering of the CLC molecules in such films.
In particular, narrow-band CLC films having left handed helical ordering selectively reflect the left hand circularly polarized (LHCP) component of incident light having wavelengths within the narrow band (e.g. 10-50) nanometers), while transmitting through the polarizer the right hand circularly polarized (RHCP) component of incident light over that narrow-band. Narrow-band CLC films having right handed helical ordering selectively reflect the RHCP component of incident light having wavelengths within the narrow band while transmitting through the polarizer the LHCP component of incident light over that narrow-band.
Similarly, broadband CLC films having left handed helical ordering selectively reflect the LHCP component of incident light having wavelengths within the band of 400-800 nanometers, while transmitting through the polarizer the right hand circularly polarized (RCP) component of incident light over that narrow-band. Broadband CLC films having right handed helical ordering selectively reflect the RHCP component of incident light having wavelengths within the band of 400-800 nanometers, while transmitting through the polarizer the LHCP component of incident light over that narrow-band.
In their native form, the prior art CLC-based (i.e. cholesteric) polarizers are limited to optical applications where either left-handed or right-handed circularly polarized light is required, as described above. However, in many applications such as LCD panels, linear polarized light is required, as described above. Consequently, circularly polarized light produced from CLC circularly polarized film must be converted to linearly polarized light. In prior art CLC polarizers, polarization conversion is carried out by passing the circularly polarized light through a quarter wave phase retardation film. Preferably, this is achieved by laminating the quarter-wave retardation film onto the surface of the CLC-based circular polarizing film. While this composite structure can be produce linearly polarizing light without absorbing photonic energy and producing heat, characteristic of dichroic linear polarizers, such a composite linear polarizer suffers from a number of shortcomings and drawbacks.
In particular, when laminating the quarter-wave retardation film onto the CLC film, the designer is constrained to use only non-birefringent adhesives, to maximize transmissions of the desired polarization state and to avoid reduction in polarizer extinction ratio. To ensure optimal light transmission through the composite structure, it is necessary to match the refractive indices of the CLC film and the quarter-wave retardation film. Also, to avoid delamination of these films, it is necessary to thermally match the coefficients of thermal expansion thereof, which is not easily achieved. Collectively, these conditions and constraints render the manufacture of such non-absorbing linear polarizers very difficult, and greatly increase the cost of manufacture of such linear polarizing structures that utilizes a quarter-wave retardation.
Thus, there is a great need in the art for a non-absorbing linear polarizer and a method and apparatus for making the same, while avoiding the shortcomings and drawbacks associated with prior art CLC-based linear polarizing structures which require the lamination of quarter-wave retardation film material to a circularly polarizing CLC film structure.